1. Field of the Invention
The invention relates to a quasi-optical gyrotron comprising first means for generating an electron beam extending in the direction of an electron beam axis, second means for generating a static magnetic field aligned in parallel with the electron beam axis, wherein due to the static magnetic field the electrons of the electron beam are forced into gyration, a quasi-optical resonator which exhibits two mirrors arranged opposite one another on a resonator axis aligned perpendicularly to the electron beam axis, in which resonator an alternating electromagnetic field is excited by the gyration of the electrons, and third means for coupling electromagnetic radiation out of the resonator.
2. Discussion of Background
A quasi-optical gyrotron of the type initially mentioned is known, for example, from Patent CH-664045 or from the article "Das Gyrotron, Schlusselkomponente for Hochleistungs-Mikrowellensender" (the gyrotron, key component of high-power microwave transmitters), H. G. Matthews, Minh Quang Tran, Brown Boveri Review 6-1987, pages 303-307. Compared with a conventional cylindrical gyrotron, this gyrotron has the advantage that it can generate high power. The reason for this lies in, among other things, the following facts;
1. Since the resonator is not coaxial but perpendicular to the electron beam axis, it can be dimensioned independently of the "electron beam part". In particular, the radiation exposure of resonator mirror and RF window can be reduced by enlarging the diameter. PA1 2. The energy present in the resonator can be coupled out via two outputs, namely at each of the two resonator mirrors.
A quasi-optical gyrotron of the type mentioned operates at frequencies of typically 150 GHz and more and can generate radiation powers of a few 100 kW in continuous mode. However, having regard to such gyrotrons being used for heating plasma in fusion reactors, continuous powers of 1 MW and more are required. Difficulties of the most varied type arise in the achievement of such high powers. One problem, in particular, is how to couple the millimeter waves efficiently out of the resonator.
From patent CH-668,865, for example, the following two proposals for coupling out of an open resonator of the type initially mentioned are known: coupling-out through annular slots in the resonator mirror and coupling-out at the edge of a mirror with a reduced diameter. However, neither of the two attempts at a solution is satisfactory. Because of the high electric field strengths occurring with high radiation powers, hazardous flash-overs can occur, on the one hand, at the edges of the slots. On the other hand, no Gaussian modes i.e., modes which yield a radiation pattern with no side lobes, are coupled out in this manner. The fact is, however, that it is only the Gaussian modes which can be transmitted without losses via a relatively long distance to a load.
It has been proposed in conjunction with the construction of high-power lasers to achieve the coupling of the radiation out of the optical resonator in such a manner that one of the two reflecting resonator mirrors is provided with a periodic structure so that about 20% of the resonator energy is coupled out laterally at a given angle. In particular, a coupling-out mirror is known from U.S. Pat. No. 3,609,585 which exhibits periodic narrow troughs which are separated by wide flat sections. The troughs in this arrangement, which are responsible for the coupling-out, are constructed to be sawtooth-shaped.
This solution known from the field of laser design, however, is not suitable for a quasi-optical gyrotron. Apart from the pointed edges, it is mainly also the wave-optical effects which are disadvantageous and which naturally already become important at relatively large dimensions (mm range) in the case of millimeter waves and lead to unwanted diffraction peaks or secondary diffraction peaks.